1. Field of the Invention
The present invention relates to a thin film transistor and a display device using the thin film transistor at least in a pixel portion.
2. Description of the Related Art
In recent years, techniques for forming a thin film transistor using a semiconductor thin film (with a thickness of about several tens to several hundreds of nanometers) which is formed over a substrate having an insulating surface have attracted attention. A thin film transistor is widely used in an electronic device such as ICs and electro-optical devices, and their development especially as a switching element for a display device has been accelerated.
As a switching element of a display device, a thin film transistor using an amorphous semiconductor film for a channel formation region, a thin film transistor using a polycrystalline semiconductor film with a crystal grain diameter of 100 nm or more for a channel formation region, or the like is used. As a method for forming a polycrystalline semiconductor film, a technique is known in which a pulsed excimer laser beam is shaped into a linear laser beam with an optical system and an amorphous silicon film is crystallized by being scanned and irradiated with the linear laser beam.
As a switching element of a display device, a thin film transistor using a microcrystalline semiconductor film with a crystal grain diameter of less than 100 nm for a channel formation region is also used (see Reference 1: Japanese Published Patent Application No. H4-242724 and Reference 2: Japanese Published Patent Application No. 2005-49832).